There are many units by which to measure the impact of climate change: degrees of increasing temperature, feet of rising sea level, dollars needed to adapt to a warming world. But a group of scientists in California have put forth an intriguing new unit of measurement: kilometers per year.

Writing in a paper published Wednesday in Nature, scientists describe what they call the velocity of climate change, or more specifically, the speed of Earth's shifting climatic zones. As global temperature rises over the next century, the scientists argue, Earth's habitable climatic zones will start moving too, generally away from the Equator and toward the poles. That means many species of plants and animals will also have to move in order to survive. Whether or not they do will depend on several factors, but two of the most important are how fast a species can adjust its habitat range, and how quickly that range is moving out from under it.

Until now, ecologists have mostly focused on these factors as they affect individual species, but the new paper takes a more global view. By combining temperature projections on a very fine scale with global topographic maps, researchers have predicted change not for specific species, but for the climatic zones they need to keep up with.

Indeed, because global temperature is rising now, ecosystems are already on the move. "Once you explain it to people, it makes intuitive sense," says co-author David Ackerly, a University of California, Berkeley, biologist. "We know what it's like to drive north to escape the heat. It's concrete, rather than the abstractness of rising average temperatures."

More than intuitive, this new index could also prove very useful, especially to conservationists who work to keep species from extinction. While the average velocity of climate change may be a bit less than a half-kilometer per year worldwide, according to the paper, it can be significantly faster or slower depending on the local topography. In deserts and other flat areas, such as the Amazon basin, climatic zones will move faster, while hilly or mountainous terrain will slow things up. "In the Northern Hemisphere, for example," explains lead author Scott Loarie, "north-facing slopes tend to be cooler and wetter than south-facing slopes."

In short, opposite sides of a mountain may have different climates, even though they're close to each other. In areas with varied terrain including lots of hills, therefore, hospitable conditions might be available relatively nearby. "That was the unexpected message," says Loarie, an ecologist at the Carnegie Institution for Science at Stanford University. "There's lots of buffering capacity in heterogeneous landscapes."

According to the velocity maps that Loarie and his colleagues put together, only 8% of the world's national parks and other preserves will retain their current climate over the next century, compounding the problem of how to keep species from going extinct. One way to do that is simply to move them. But that's not only extraordinarily difficult, it can also backfire - just ask anyone in the southeastern U.S. about the inexorable advance of the imported invasive species the kudzu vine. "For some species on the brink of extinction, physically moving them might be our only option," says Loarie, "but setting aside connected, heterogeneous landscapes that allow natural movement will almost certainly be an better use of conservation dollars."

He and the other co-authors, including scientists at Climate Central in Palo Alto and the California Academy of Sciences in San Francisco, emphasize that their velocity maps are oversimplifications - at least so far. For one thing, they do not account for the unique characteristics of various species within a given ecosystem. Some species may have more tolerance for climate changes than others, and may not need to move as quickly; some species may be intolerant of change but unable to move. Other species may be sensitive to changes in rainfall, while still others responsive only to temperature - and changes in these weather patterns may not happen at the same rate. "The complexity is daunting," says Ackerly.

Nevertheless, while the climate-velocity concept is still crude, it's promising enough that Ackerly is collaborating with an organization called the Bay Area Open Space Council on habitat conservation strategies in central California. The new research informs one of the key challenges conservationists face: having only limited funds to buy up land, and, thus, having to spend wisely. "What we bring," says Ackerly, "is the ability to think about how topography might affect those decisions."

用以衡量气候变化影响的单位有很多：气温上升的度数，海平面上升的尺数，为适应这个在变暖的世界所需支付的金钱数。然而在美国加州，一些科学家们新发现了一个有趣的衡量单位：千米数每年。

在周三版《自然》杂志上的一篇文章中，科学家描述了他们所谓的气候变化速率，更确切地说，是地球变更其气候带的速度。科学家表示，在下一世纪中，随着全球气温不断上升，地球上适宜居住的气候带也会有所变动，逐渐远离赤道向两极发展。这就意味着许多动植物物种若要继续存活就必须迁移。它们是否能够存活与诸多因素密切相关，其中最重要的两点是：该物种调整其栖息地范围的速度，以及该范围自身变化的速度。

在这以前，生态学家大多关注气候变化的各种因素对个体物种的影响，但是这篇文章更加综合地从整体的视角看问题。通过将气温变化预测很好地和全球地形图相结合，研究人员预言变化不会是个别物种的，气候带也将产生变化，这也是他们需要跟进的。

事实上，由于全球气温正在不断上升，生态系统早已经开始变化了。"一旦你向人们解释这个现象，它就有了直观的意义。"合着者戴维·阿克利，来自加州柏克莱大学的一位生物学家说道，"我们知道向北发展以躲避气温的上升是怎么一回事。它是具体的，而非抽象的平均气温上升。"

除了直观以外，这项新的指数也被证实是非常有用的，尤其是对那些致力于保护物种免于灭绝的环保主义者们来说。世界范围内气候变化所引起的气候带平均变化速率是略小于0.5千米每年，然而文章还指出，由于各地地形不同，该数字很可能会有较大幅度的浮动。在沙漠地区以及其他平原地区，如亚马逊盆地，气候带的变化会更快，相对而言，山区、丘陵地区比较缓慢。"比方说，在北半球，"主笔斯科特·劳瑞解释到，"朝北的山坡往往比朝南的阴凉潮湿。"

简言之，即便彼此靠得很近，一座山峰的两侧也可能会有不同的气候。因此，地形多变的地区，包括丘陵地带，在相对其比较近的地方仍然可能找到宜人的环境。斯坦福大学卡耐基科学院的生态学家劳瑞说："我们没有预料到是。在这多样化的环境中其实还有很多的缓冲能力存在。"

劳瑞和他的同事们绘制的变化速率地图显示，世界上仅有8%的国家公园以及其他自然保护区能在下个世纪保持其现有的气候，这也加剧了如何防止物种灭绝这一问题的严重性。解决方案之一就是迁移那些面临濒危的物种。然而那样做不仅异乎寻常地困难，而且可能产生事与愿违的结果--比如美国东南部人尽皆知的葛藤，该植物在被引进后攻击性地大肆蔓延。"对一些濒临灭绝边缘的物种进行物理迁移可能是我们唯一的选择，"劳瑞说，"但是那些允许自然迁移的环境多样性环境相互连接，如果撇开这些，那几乎肯定是节省资金的更好的方法。"

他以及其他的合着者们，包括帕洛阿尔托气候研究中心和旧金山加利福尼亚科学院的科学家们，他们强调指出，该变化速率图尚有些过度单纯化，至少目前是这样的。首先，他们没有对给定生态系统中各物种的自身特征做出说明。有一些物种可能比其他的物种对气候变化具有更强的适应性，就不需要迅速转移；还有一些物种虽然适应性也强，但可能没法迁移。一些物种可能在雨季会对变化非常敏感，而另一些只会对温度敏感--并且气候的变化形式很可能不会总是以相同的比率发生。阿克利说："其复杂性令人望而生畏。"

尽管气候变化速率的概念尚未成熟，但是目前阿克利正于一个叫做海湾地区绿地委员会的合作，他们致力于研究加利福尼亚中部地区居住地的环境保护战略，这是非常振奋人心的。新的研究指出环保主义者面临的主要挑战在于：能用来尽量收购土地的资金非常有限，因此必须理智地花钱。阿克利说："我们能够提供的是有能力去思考地形会如何影响到这些决策。"